Process for explosively bonding metal layers



y 1966 A. H. HOLTZMAN PROCESS FOR EXPLOSIVELY BONDING METAL LAYERS Filed Jan. 25, 1963 N m M Z T T N m E V H m 2 H D la m mun i r Q s E j r 2 y 2 O 7/ m United States Patent 3,261,038 PROCESS FOR EXPLOSIVELY BONDING METAL LAYERS Arnold H. Hoitzman, Cherry Hill Township, N.J., as-

signor to E. I. du Pont de Nemours and Company,

Wilmington, Del., a corporation of Delaware Filed Jan. 23, 1963, Ser. No. 253,484

7 Claims. (Cl. 29-486) The present invention relates to an improved method for bonding metals by explosive means.

Copending applications Serial Number 65,194, now US. Patent 3,137,937, teaches a process for bonding metal layers to form a multilayered body by supporting a metal cladding layer a distance of at least 0.001 inch from a metal base layer, placing a layer of an explosive having a detonation velocity less than 120% of the sonic velocity of the metal with the highest sonic velocity in the system on the outside surface of the cladding layer and initiating the explosive so that detonation is propagated parallel to the metal layers.

Although the bonding obtained by this process is excellent, under certain circumstances some areas around the edges of the clad metal system are left unbonded.

This problem can be alleviated and essentially complete bonding along edges of the clad metal system can be insured by the process of the present invention which comprises: 7 (l) supporting a metal cladding layer directly above, substantially parallel to, and separated a distance of at least 0.010 inch from an equidimensional metal base layer,

(2) covering the exposed outside surface of the metal cladding layer with a layer of a first detonating explosive, said first explosive having a detonation velocity of between about 1200 meters per second and 100% of the sonic velocity of the metal having the highest sonic velocity in the system,

(3) positioning a linear charge of a second detonating explosive along a median line extending across said layer of a first explosive, said second explosive having a detonation velocity greater than the detonation velocity of said first explosive and less than 120% of the sonic velocity of the metal having the highest sonic velocity in the system, and

(5) initiating said linear charge at a single point on said charge.

The term metal as used in this application includes any solid elemental metal or solid mixture of elemental metals, e.g., an alloy.

The term cladding layer refers to that metal layer which is explosively bonded to an equidimensional metal base layer by the process of the present invention, the term equidimensional being defined herein as having the same length and width. Both the cladding and base layers are substantially rectangular and can be of the same, or generally, different thickness.

The term sonic velocity as used throughout this application in connection with metals and metallic systems refers to the velocity of the plastic shock wave which forms when a stress which is applied just exceeds the elastic limit for unidimensional compression of the particular metal or metallic system involved.

By the term linear charge is meant a charge of explosive having a length substantially greater than its width or thickness and having a relatively uniform transverse cross-section. The width of the linear charge can be as great at of the width of the layer of explosive. However since uniform metallurgical bonding between the cladding and base layers is best effected by a uniform layer of explosive, the size and explosive loading of the linear charge is kept to the minimum neces- 3,261,088 Patented July 19, 1966 sary to effect initiation of the uniform layer. The optimum size and explosive loading of the linear charge which should be used to effect reliable initiation of the layer explosive without substantially increasing the overall explosive loading on the cladding layer and thus causing deformation of the clad metal system is obvious to one skilled in the art. A linear charge is distinguished from, for example, a sheet of explosive, or a lump of explosive. The linear charge can be self supporting, e.g., an extruded cord of a flexible explosive composition or a thin strip cut from a sheet of such a composition, a loose granular explosive or a gelatinous composition contained in a metal, plastic, paper, or textile tube or sheath, a cast explosive, etc. Alternatively, the linear charge can be provided, for example, in the form of a loose, granular composition distributed or poured through a funnel or some other device along a median line across the layer of a first explosive. The linear charge generally is placed on top of the layer of explosive but it can be placed on a median line of the layer of explosive below the layer of explosive. Alternatively the layer of explosive can be split along a median line and the linear charge placed between the two halves of the layer of explosive.

The term median line refers to a line which extends from the center of one edge of the layer of a first explosive to the center of the opposite edge of the layer.

For a more complete understanding of the process of the present invention reference is now made to the drawings in which FIGURE 1 and FIGURE 2 illustrate perspective views of preferred embodiments of cladding assemblies which may be used for the practice of the present invention.

In FIGURE 1, metal cladding layer 1 is supported above metal base layer 2 (supporting means not shown). The upper or exposed surface of cladding layer 1 is covered with a layer of a detonating explosive 3 having a detonation velocity between about 1200 meters per second and of the sonic velocity of the metal with the highest sonic velocity in the clad system. A linear charge of a detonating explosive 4 having a detonation velocity higher than the detonation velocity of the layer of explosive and less than 120% of the sonic velocity of the metal with the highest sonic velocity in the clad system to which is attached an electric blasting cap 5 having lead wires 6 to a source of electric current is positioned on the median line of the layer of explosive 3.

The cladding process can be carried out in a vertical arrangement as well as in the horizontal arrangement shown in the drawings and the means of supporting the metal cladding layer separated from the metal base layer is not critical to the present invention. The minimum separation between the layers which results in effective bonding is about 0.010 inch. If the detonation velocity of the linear explosive charge exceeds 120% of the sonic velocity of the metal with the highest sonic velocity in the system, oblique shock waves often ensue which prevent formation of a strong, continuous metal-to-metal bond between the metal layers. If the detonation velocity of the layer of explosive is below 1200 meters per second, the explosive fails to develop the energy necessary to firmly bond the metals within the sense and scope of the invention. In all cases the detonation velocity of the linear charge is higher than that of the explosive layer. Generally there is a considerable difference between the detonation velocity of the two explosives and the process as described above wherein the detonation velocity of the linear charge is between about ofthe detonation velocity of the explosive layer and of the sonic velocity of the metal with the highest sonic velocity in the system represents a preferred embodiment of the invention.

Effective bonding within the sense and scope of this invention will not be obtained the metal cladding layer is propelled in gross against the metal base layer by travelling in a direction generally normal to the surface of the metal base layer. In other words, the two metal layers must make contact at an angle in order to insure formation and/or circulation of the jet which is responsible for the formation of the bond between the layers.

In order to establish this angular configuration and to insure uniform bonding in the zone directly below or contiguous to the linear charge, the linear charge must be initiated at a single point. If the linear charge is initiated at a plurality of points interfering shock fronts which exert a deleterious effect on uniform bonding and often cause deformation of the cladding system ensue. If the linear charge is initiated simultaneously over its entire length that portion of the metal cladding layer which is directly below or contiguous to the median line of the layer of explosive upon which the linear charge is placed is propelled in gross against the base layer and a linear unbonded zone in the clad metal system results. The linear charge usually is initiated at one end or in the center of the charge by any convenient means of initiation such as an electric blasting cap, deton-ating cord, etc.

Although it is not intended to be limited by any theory of operation it is believed that in substantially rectangular systems in which the layer of explosive is initiated at a point or points along one edge of the system unbonded zones along the two edges which are perpendicular and adjacent to the initiation edge are attributable at least in part to the shape of the detonation front, to the acute angle at which the explosive pressure is applied to these edges of the cladding layer, and to the entry of gaseous detonation products into the space between the metal layers. In the process of the present invention the explosive layer is initiated by means of a linear charge having a detonation velocity greater than that of the explosive layer and still below the upper limit for effective cladding. Thus the explosive layer is initiated along the median line of the layer and detonation proceeds outward from this line or, in other Words, substantially normal to these edges. When the detonation front reaches each of these edges it is substantially parallel to the edge; gaseous detonation products are prevented from entering the space between the metal layers, and the explosive pressure is applied to these edges in the optimum configuration for effective bonding.

Since the use of the linear charge of explosive is effective in providing essentially complete bonding only on those edges of a rectangular clad metal system which are parallel to the linear charge, it is often desirable to use metal extension pieces as taught in copending United States application Serial No. 253,485, filed January 23, 1963, on one or both of the edges of the cladding layer which are perpendicular to the linear charge.

An assembly which can be used for the practice of the present invention is illustrated in FIGURE 2 which differs from FIGURE 1 in that the layer of a detonating explosive 3 covers the area defined by the upper surfaces of metal cladding layer 1 and metal extension piece 7 which is attached to one edge of metal cladding layer 1.

Any convenient method for attaching the extension piece(s) to the cladding layer can be used, i.g., epoxy resin, shallow metallic weld, etc. However, the joint between the extension piece(s) and the edge(s) to which it is joined should be of such strength that it breaks under the pressure from the detonation of the explosive layer thus causing the extension piece(s) to be sheared from the assembly during the cladding process.

The process of the present invention is particularly useful when the cladding and base layers are rectangular metal sheets, plates, or strips having considerably greater length than width. A linear charge of explosive positioned on that median line of the layer of explosive which is parallel to the length of the cladding layer and initiated .at the midpoint of the charge insures essentially complete bonding along the two long edges of the rectangular clad metal system. If the metals comprising the cladding and base layers are relatively inexpensive the loss involved in trimming any narrow unbonded zones along the short edges of the clad system is insignificant. However, if, for

' example, the cladding layer comprises gold or some other precious metal, metal extension pieces attached to the two short edges of the cladding layer as taught in the abovementioned copending United States application Serial No. 253,485, insure essentially complete bonding along these edges. An alternative arrangement for cladding with an assembly having considerably greater length than width wherein the cladding layer in relatively thin involves initiating the linear charge at one end using a single extension piece attached to the adjacent short end of the cladding layer to counteract the anomalous edge effects associated with that edge of the cladding layer contiguous to the point of initiation. In any case the linear charge of explosive insures essentially complete bonding along the long edges of the clad metal system to which it is parallel and obviates the need for attaching extension pieces to these edges of the clad-ding layer. This constitutes a considerable saving since the process of attaching extension pieces is tedious and time consuming and, in any case, the extension pieces are grossly deformed during detonation and represent an unavoidable material loss.

As is obvious to one skilled in the art, a great variety of explosive compositions may be used within the sense and scope of this invention. Among the suitable compositions for use in the explosive layer are grained amatol explosives comprising mixtures of ammonium nitrate and trinitrotoluene in various proportions, mixtures of these amatol explosives with inert diluents such as sodium chloride and with sodium nitrate, and mixtures of ammonium nitrate and fuel oil in various proportions. The linear charge may comprise, for example, a strip or cord of one of the number of fibrous and flexible explosive compositions based on pentaerythritol tetranitrate and other crystalline explosives. The use of a self-supporting linear charge represents a preferred embodiment of the invention.

The following examples illustrate some of the modifications of the process of the present invention. They are intended as illustrative only, however, and are not to be considered as exhaustive or limiting.

Example 1 A completely bonded stainless steel-on-mild steel clad metal system is prepared in the following manner. An austenitic stainless steel plate A; inch thick, 6 inches wide, and 9 inches long is placed on top of a mild steel plate /2 inch thick, 6 inches wide and 9 inches long. A spacing of .060 inch between the adjacent surfaces of the two plates is provided by placing shim steel heads inch in diameter and .060 inch deep on each of the corners of the mild steel plate and spot welding them in place. A piece of mild steel A; inch thick, 1 inch wide and 6 inches long is butted against one of the 6-inch long edges of the stainless steel plate and glued in place with epoxy resin. A Mr-inch layer of a grained amatol explosive comprising 50 parts ammonium nitrate and 50 parts trinitrotoluene is spread over the upper surfaces of the stainless steel plate and the mild steel extension piece and kept in place with a wooden frame positioned around the perimeter of the layer of explosive. The detonation velocity of the amatol explosive is about 3800 meters per second. A piece 10 inches long and about .18 inch in diameter of an explosive cord comprising 24 parts pentaerythritol tetranitrate, 67 parts red lead, 2.25 parts refined mineral oil, 2.36 parts polybutene, 1.69 parts polyisobutylene, 1.35 parts butyl rubber, and 1.35 parts aromatic hydrocarbon resin plasticizer and having a detonation velocity of about 4370 meters per second is placed on top of the layer of amatol explosive substantially as illustrated in FIGURE 2. An electric bl-asting cap having lead wires to a source of electric current is attached to the end of the explosive cord as shown in FIGURE 2. The blasting cap is actuated by application of electric current and initiates, in turn, the explosive cord and the layer of amatol explosive. After detonation the clad metal system is recovered. The mild steel extension piece is sheared off and ultrasonic testing reveals complete bonding along the edges of the clad metal system adjacent to the edge to which the extension piece is attached.

A second stainless steel-on-mild steel clad is made using the same materials and conditions with the single exception that initiation is accomplished by means of the electric blasting cap alone. No explosive cord, i.e., no linear charge, is used. Ultrasonic testing reveals unbonded zones about 1 inch wide along the entire length of the edges adjacent to the initiation edge.

Example 2 A stainless steel-on-rnild steel clad metal system 6 inches long and 6 inches wide is made using the materials and technique described in Example 1. However, -in this case, mild steel extension plates are attached to two opposite edges of the stainless steel plate. The explosive cord is 11 inches long and is positioned down the center of the layer of amatol explosive extending from the free 6-inch edge of one extension piece to the free 6-inch edge of the other extension piece. After detonation the clad metal system is recovered. Both extension pieces are sheared 011 and ultrasonic testing reveals complete bonding along all edges of the cl-ad metal system including those adjacent to the edges to which the extension pieces are attached.

Example 3 A stainless steel-on-mild steel clad metal system 18 inches long and 6 inches wide is made using the materials and a modification of technique described in Example 1. However, in this case no mild steel extension pieces are attached to the stainless steel plate. The explosive cord is 18 inches long and is positioned down the center of the layer of amatol explosive extending from one 6- inch edge to the other 6-inch edge of the layer. The cord is initiated in the center by means of an electric blasting cap and after detonation ultrasonic testing reveals no unbonded zones along the 18-inch edges of the clad metal system.

Example 4 A copper plate A1. inch thick and 24 inches long and 6 inches wide is clad to a mild steel plate /2 inch thick and 24 inches long and 6 inches wide using the technique described in Example 3. The spacing between the copper and mild steel plates prior to detonation is .25 inch and the layer of amatol explosive is 1% inches thick and has a detonation velocity of about 4100 meters per second. The explosive cord is 24 inches long. No metal extension pieces are used. Ultrasonic inspection reveals complete bonding along the two 24-inch edges of the copper-mild steel clad metal system.

A second copper-on-mild steel composite is made exactly as described above with the single exception that initiation is accomplished by means of an electric blasting cap alone. No explosive cord, i.e., no linear charge, is used. Ultrasonic inspection reveals unbonded zones around the entire perimeter of the composite extending approximately 2 inches in from the edges.

Example 5 A completely bonded titanium-on-carbon steel clad metal system is prepared in the following manner. A titanium plate inch thick, 6 inches wide, and 9 inches long is positioned above and parallel to a carbon steel plate /2 inch thick, 6 inches wide, and 9 inches long. The adjacent surfaces of the two plates are separated a distance of .060 inch maintained by shim steel cups 4; inch in diameter and .060 inch deep which are placed 2 inches apart in 3 rows on the carbon steel plate. A piece of carbon steel A inch thick, 1% inches wide and 6 inches long is butted along its length against one of the 6-inch edges of the titanium plate and glued in place with aluminum-filled epoxy resin. A A-inch layer of a grained amatol explosive comprising /2 part fine alumina and the remainder, a /30 mixture of trinitrotoluene and ammonium nitrate and having a detonation velocity of about 4000 meters per second is spread over the upper surfaces of the titanium plate and the carbon steel extension piece and kept in place with a wooden frame positioned around the perimeter of the layer of explosive. A strip /2 inch wide and 10 /2 inches long of a fibrous explosive composition comprising parts pent-aerythritol tetranitrate, 17.5 parts of an acrylonitrile/butadiene rubber and 7.5 parts paper pulp is placed on top of the layer of amatol explosive substantially as illustrated in FIGURE 2. The weight distribution of explosive ingredients in the fibrous composition is 1 gram per square inch and the composition has a detonation velocity of about 4450 meters per second. An electric blasting cap is attached to the end of the explosive cord substantially as shown in FIGURE 1. The blasting cap is actuated by application of electric current and initiates, in turn, the explosive cord and the layer of amatol explosive. After detonation, the carbon steel extension piece is sheared from the titanium plate and the two plates are uniformly, metallurgically bonded together over the entire area of the interface between the plates. Ultrasonic probing reveals no unbonded zones or bond defects along the edges of the composite adjacent to the edge to which the extension piece is attached.

The invention having been fully described in the foregoing we intend to be limited only by the following claims.

What is claimed is:

1. A process for preparing a metallurgically bonded clad metal system which comprises:

(1) supporting a metal cladding layer directly above, substantially parallel to, and separated a distance of at least 0.010 inch from an equi-dimensional metal base layer,

(2) covering the exposed outside surface of the metal cladding layer with a layer of a first detonating explosive, said first explosive having a detonation velocity of between about 1200 meters per second and of the sonic velocity of the metal having the highest sonic velocity in the system,

(3) placing a linear charge of a second detonating explosive along a median line extending across said layer of a first explosive, said second explosive having a detonation velocity greater than the detonation velocity of said first explosive and less than 120% of the sonic velocity of the metal having the highest sonic velocity in the system, and

(4) initiating said linear charge at a single point on said charge.

2. A process as in claim 1 wherein the detonation velocity of said second explosive is between about of the detonation velocity of said first explosive and of the sonic velocity of the metal having the highest sonic velocity in the system.

3. A process as in claim 1 wherein said first explosive comprises a mixture of ammonium nitrate and trinitrotoluene.

4. A process as in claim 1 wherein said linear charge is self-supporting.

5. A process as in claim 1 wherein said linear charge comprises as the major explosive component, pentaerythritol tetranitrate.

6. A process as in claim 1 wherein said metal cladding and base layers are substantially rectangular, metal extension pieces are attached to at least one of the two edges of the metal cladding layer which are perpendicular to said linear charge, and the area defined by the upper surface of said metal cladding layer and metal extension pieces is covered with said layer of a first detonating explosive.

7. In the process for the preparation of a metallurgically bonded clad metal system by (l) supporting a metal cladding layer directly above, substantially parallel to and separated a distance at least 0.010 inch from an equidimensional metal base-layer,

(2) covering the exposed outside surface of the metal cladding layer with a layer of a first detonating explosive, said first explosive having a detonation velocity of between about 1200 meters per second and 100% of the sonic velocity of the metal having the highest sonic velocity in the system, and

(3) initiating said layer of a first detonating explosive so that detonation is propagated parallel to said metal cladding layer,

the improvement which comprises placing a linear charge of a second detonating explosive along a median line extending across said layer of a first explosive, said second explosive having a detonation velocity greater than the detonation velocity of said first explosive and less than 120% of the sonic velocity of the metal having the highest sonic velocity in the system and initiating said linear charge at a single point on said charge so that said linear charge initiates said layer of a first detonating explosive.

References Cited by the Examiner UNITED STATES PATENTS JOHN F. CAMPBELL, Primary Examiner. 

1. A PROCESS FOR PREPARING A METALLURGICALLY BONDED CLAD METAL SYSTEM WHICH COMPRISES: (1) SUPPORTING A METAL CLADDING LAYER DIRECTLY ABOVE, SUBSTANTIALLY PARALLEL TO, AND SEPARETED A DISTANCE OF AT LEAST 0.010 INCH FROM AN EQUI-DIMENSIONAL METAL BASE LAYER, (2) COVERING THE EXPOSED OUTSIDE SURFACE OF THE METAL CLADDING LAYER WITH A LAYER OF A FIRST DETONATING EXPLOSIVE, SAID FIRST EXPLOSIVE HAVING A DETONATION VELOCITY OF BETWEEN ABOUT 1200 METERS PER SECOND AND 100% OF THE SONIC VELOCITY OF THE METAL HAVING THE HIGHEST SONIC VELOCITY IN THE SYSTEM, (3) PLACING A LINEAR CHARGE OF A SECOND DETONATING EXPLOSIVE ALONG A MEDIAN LINE EXTENDING ACROSS SAID LAYER OF A FIRST EXPLOSIVE, SAID SECOND EXPLOSIVE ING A DETONATION VELOCITY GREATER THAN THE DETONATION VELOCITY OF SAID FIRST EXPLOSIVE AND LESS THAN 120% OF THE SONIC VELOCITY OF THE METAL HAVING THE HIGHEST SONIC VELOCITY IN THE SYSTEM, AND (4) INITIATING SAID LINEAR CHARGE AT A SINGLE POINT ON SAID CHARGE. 